input - source index, can be read-only, can have deletions, can have
multiple segments (or multiple readers).

outputs - list of directories where the output parts will be stored.

seq - if true, then the source index will be split into equal
increasing ranges of document id-s. If false, source document id-s will be
assigned in a deterministic round-robin fashion to one of the output splits.

split

input - source index, can be read-only, can have deletions, can have
multiple segments (or multiple readers).

outputs - list of directories where the output parts will be stored.

seq - if true, then the source index will be split into equal
increasing ranges of document id-s. If false, source document id-s will be
assigned in a deterministic round-robin fashion to one of the output splits.